1. Field of the Invention
The present invention is directed to spark plugs of internal combustion engines, and more particularly, to spark plugs including high temperature performance electrodes.
2. Description of the Prior Art
Spark plugs are widely used to initiate combustion in an internal combustion engine. Spark plugs typically include a ceramic insulator, a conductive shell surrounding the ceramic insulator, a central electrode disposed in the ceramic insulator, and a ground electrode operatively attached to the conductive shell. The electrodes each have a sparking end, such as a tip, disk, rivet, or other shaped portion. Each sparking end presents an outer surface, including a spark contact surface. The spark contact surfaces of the sparking ends are typically exposed planar surfaces located proximate one another and defining a spark gap therebetween. Such spark plugs ignite gases in an engine cylinder by emitting an electrical spark jumping the spark gap between the central electrode and ground electrode, the ignition of which creates a power stroke in the engine.
Due to the nature of internal combustion engines, spark plugs operate in an extreme environment of high temperatures of at least 500° C. and various corrosive combustion gases which has traditionally reduced the longevity of the spark plug. The sparking ends or material adjacent the sparking ends of the electrodes also experience electrical erosion due to localized vaporization resulting from high arc temperatures of the electrical arc during operation of the spark plug. The electrodes may also experience growth of various particulates and oxidation, particularly at the sparking ends. Over time, the electrical spark erosion, particulates, and oxidation reduces the quality of the spark between the center electrode and ground electrode, which in turn affects the performance of the spark plug, and the resulting ignition and combustion.
Existing spark plug electrodes are often formed of a nickel (Ni) material, such as pure Ni or Ni alloys having high resistance to corrosion and oxidation. However, such Ni electrodes experience a significant amount of electrical spark erosion which limits their use in spark plugs.
In attempt to reduce the amount of electrical spark erosion and improve the performance of Ni electrodes, sparking ends formed of precious metal materials have been attached to a base formed of Ni material. The precious metal material is typically a platinum (Pt) material, such as pure Pt or alloys thereof. The sparking ends formed of the Pt material have a low electrical spark erosion rate and thus improve the performance of the electrode. However, the high cost of such precious metals limits their use throughout the entire electrode.
Further, the use of a Pt material in the sparking ends is limited because Pt materials experience balling or bridging due to excessive oxidation upon exposure to sparks and the extreme conditions of a combustion chamber. FIGS. 7 shows prior art sparking ends formed of a Pt alloy and including metal balls formed at the sparking ends. The metal balls typically grow over time and may bridge the spark gap between the central electrode and ground electrode. The bridging typically hinders the performance of the electrodes, which in turn affects the resulting ignition and combustion, including the power output, fuel efficiency, performance of the engine, and emissions.
Sparking ends have also been formed of Iridium (Ir) material, such as pure Ir or alloys thereof. The Ir materials do not experience the balling or spark erosion experienced by the Ni materials and Pt materials. However, the use of Ir materials is limited because such materials experience corrosion in the presence of calcium (Ca) and phosphorus (P). Ca and P are often present in engine oils and oil additives, which the sparking ends are exposed to during operation of the spark plug in an internal combustion engine. Recently, increasing amounts of Ca and P are found in combustion materials as engine manufacturers attempt to reduce friction to increase fuel economy by alloying more engine oil to seep into the combustion chamber.